a) Field of the Invention
The invention concerns an actuator for converting rotary movement into linear movement. The actuator comprises a prime mover turning a strain wave reduction gear assembly which, in turn, drives a mechanical transmission including a linearly moving control element.
It is known to use actuators to compensate for the bending of heavy and/or pre-loaded structural members in the headbox or coating sections of paper-making machines. Further, it is also known to use actuators for adjusting the blades of plastic extrusion machines, as well as veneering machines.
b) Description of Related Art
European Patent EP-0 232 022 discloses an actuator for converting relatively low torque rotary movement into relatively high force linear movement. Specifically, a prime mover turns a first strain wave reduction gear assembly. In turn, the first strain wave reduction gear assembly turns a second strain wave reduction gear assembly which drives a spindle constrained to linear movement via acme threads cooperating with a peg and slot.
Further, actuators for converting low torque rotary movement into high force linear movement using electric motors driving either a worm gear or a planetary gear acting on a threaded spindle are known.
In these known actuators, linear positioning is regulated using closed loop feedback control. When the desired position has been reached, the actuator is switched "off" (i.e. the actuator is disconnected from a current supply) and the control loop is deactivated. However, it becomes necessary to provide self-locking elements in the reduction gear assembly to retain the desired position within one micron.
Disadvantages of the known actuators include unsatisfactory positioning accuracy and poor resolution. A combination of hysteresis effects due to friction, mechanical play, wind-up and stick/slip result in poor movement characteristics, which in turn prevent micron-range positioning precision. Additionally, the known actuators suffer from excessive wear of gear elements such as worm gears and acme threaded spindles. Further, many of the known actuators are pre-loaded to remove mechanical backlash, which also leads to increased wear of the gear elements. It has also been found that the power consumption and heat generated by the known actuators is excessive in some application.
Another significant disadvantage of the known actuators is the wide variation in the degree of hysteresis loss as a function of the control element's position with respect to the actuator. Accordingly, it is not possible to relate the linear movement force to the power consumption of the prime mover for the purpose of regulating the force based upon power consumption. Instead, overload protection devices such as the mechanical overload clutch disclosed in EP-0 232 022 are used to protect the gear elements and the control loop components.